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Pulse Detector Circuit

Lecause

New Member
Hello Guys,

I'm trying and failing to find a product off the shelf, so i thoguht i mgiht try to design soemthing to suit. What i need is a circuit that would achieve the following:

- Input is a square wave then output is high.
- input is constant (constant LOW or constant HIGH), then output is low.

The input signal would be switching from 0V to 3.3V, and its frequency can be adjusted to suit, but would be pretty low (up to 20 HZ, the lower the better).
The output should be 3.3V or 0V as well.
The circuit should raw 16mA max
 

AnalogKid

Well-Known Member
Most Helpful Member
This is called a missing-pulse detector. The 3.3V power source limits your component choices a bit, but it can be done with a 555, a CMOS gate like a CD4093, a low voltage comparator, or a couple of discrete transistors. Note that this type of circuit is based on a timer of some kind, so there is an inherent delay in the output. The circuit must wait longer than the longest possible period of the input waveform to make sure another input pulse isn't coming. You have to know the lowest possible operating frequency to design around.

What is the duty cycle of the input square wave (% time high, % time low)?

What is the longest possible time period between successive input positive-going edges?

ak
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Regardless of the actual method, you need to know how long it has to be before it 'decides' there are no pulses - and this obviously depends entirely on the exact purpose of the circuit.
 

gophert

Well-Known Member
Most Helpful Member
Regardless of the actual method, you need to know how long it has to be before it 'decides' there are no pulses - and this obviously depends entirely on the exact purpose of the circuit.
My guess is it is a 10Hz signal and he needs to know that it is "in constant mode" about 25 milliseconds after it goes low.
.
.
 

Lecause

New Member
Thank you all for the replies! The signal would just be a 10 Hz square wave (0 to 3.3v) with 50% duty cycle.
 

Lecause

New Member
I looked into using a 555 timer but i can't get the circuit to work. So far it seems the output stays on if the input is a constan 0V but goes low if the input is high.
 

alec_t

Well-Known Member
Most Helpful Member
Post deleted
 
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
i found this in another thread and have been trying to alter it to replicate my application. I've changed the input signal to 0V - 3.3V with 10Hz frequency and have been adjusting the components to suit.
http://www.falstad.com/circuit/e-555missing.html
I can't say I'm terribly happy with that circuit, I'd prefer an NPN transistor, and no logic gate.

However, in either case they don't meet your requirements as they don't detect a missing pulse if the input stays high, only if it stays low.
 

crutschow

Well-Known Member
Most Helpful Member
Here's the LTspice simulation of a missing-pulse-detector circuit using a Schmitt trigger CMOS buffer gate.
The input is capacitively coupled, so the output will go high for missing pulses, whether the input stays high or low.
The Schottky diodes peak rectify the input signal to give a low voltage on the gate input.
The response time is determined by the R1C2 time-constant.

1554765137245.png
 

Lecause

New Member
Thank you so much Zapper! This looks ideal. I'll build it on LTspice too and tune the components to suit!
 

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